Green Home Performance
Successful green building means more than using green materials. Another key to successful green building is the performance of the home: How much energy does the home use? How comfortable and healthy is the indoor environment for the homeowners?
Energy-efficient homes are comfortable, healthy homes. Energy efficiency in a home is affected by the thermal enclosure, including insulation and air sealing, as well as by HVAC size, design and type of installation. Certainly, an overriding majority of homes have all these components, but to achieve maximum energy efficiency for the home as an integrated system, it's critical to optimize material and design choices and install all components correctly.
The thermal enclosure includes the components of the home that isolate it from heat gain and loss and from air leaks. They include the structure, insulation, exterior finishes, sheathing, drywall, doors and windows. A properly installed thermal enclosure not only can reduce energy consumption but can also alleviate customer complaints about drafts or uneven temperatures. The HVAC system can't be expected to work efficiently if the thermal enclosure is inadequate. Deficiencies in the thermal enclosure—uncontrolled air leakage, improperly installed or missing insulation, minimum efficiency windows—contribute to uncomfortable conditions in the home. In most cases, oversizing the HVAC system won't make up for these problems.
Installing insulation incorrectly reduces the effective R-value of the wall systems and can lead to "hot spots" or "cold spots" in the home. This requires the HVAC equipment to work harder, leading to higher energy bills and variable temperatures throughout the home. Here's a breakdown of the most popular insulation types for residential construction, with installation tips for each:
- Blanket insulation. Don't leave gaps within the wall cavity; be especially careful to avoid gaps around obstructions such as electrical outlet boxes and plumbing pipes. Also, don't compress the insulation between framing members, as this allows more air circulation/transfer within the wall cavity, reducing the R-value performance.
- Blown insulation. Spray it at the correct density and to the correct depth to achieve the required R-value. Be sure to blow insulation into all areas of the attic, as well as around and behind framing obstructions.
- Rigid foam board. Cut it neatly to fit around obstructions without leaving gaps.
- Expanding foam. Hire an installation specialist to prevent gaps and avoid distorting other building materials.
The goal of air sealing is to create a continuous barrier around the conditioned living space against unwanted airflow. Unwanted airflow equates to unwanted loss of conditioned air and gain of unconditioned air. For the most part, drywall, interior sheathing and floor decking create a continuous air barrier between indoors and outdoors, especially if these materials are glued to the framing. Penetrations in the air barrier around wiring, plumbing, ductwork and other features, however, do allow unwanted airflow and must be eliminated.
To stop air leaks at seams and around penetrations, seal these areas with low-expanding foams, foam strips, weather stripping, weatherproof tape or caulks. Successful air sealing doesn't require special construction skills, although for optimum control, gaps must be sealed throughout the construction process: during framing, before insulating, before installing drywall, after installing drywall and interior finishes, after installing fixtures, and during the final punch-out.
HVAC: Sizing, Design, Installation
At a project’s outset, your HVAC contractor must accurately calculate the home's heating and cooling loads according to the procedures in the following publications, as established by the Air Conditioning Contractors of America (ACCA) and approved by the American National Standards Institute (ANSI):
- Manual J: Residential Load Calculation Procedure. Used to properly size and select HVAC equipment in residential homes.
- Manual S. Used to select mechanical equipment to meet Manual J loads based on local climate and ambient conditions at the building site.
- Manual D. Used to size ductwork for both constant-volume systems and zoned, variable-air-volume systems over the full range of duct-construction materials.
Properly sized heating and cooling equipment will run more frequently, helping to mix the air in the home and provide more uniform temperatures. Also, a correctly sized air conditioner will run longer, reducing the evaporator coil temperature and removing moisture from the air; drier air reduces the likelihood of mold growth in the ductwork.
Design of the distribution system (i.e., sizing of ductwork and location of supply resisters) is important for delivering conditioned air throughout the home and ensuring that it mixes well with the home's existing air. Correct air distribution minimizes temperature stratification and eliminates drafts from the supply registers.
Finally, overall quality control must be rigorous during installation of the HVAC system or all the design and engineering effort is pointless. Be sure all ductwork is located within the thermal enclosure to prevent it from being exposed to outdoor temperatures. It also should be sealed with mastic to prevent air leaks. Make your HVAC contractor explicitly aware of all installation details, and continually review work in progress to verify correct installation.